But one source tells us Selvadurai and Crowley "hadn't been getting along" for some time now.

Another source close to Selvadurai tells us he has, for a while now, felt lost at Foursquare and he has been frustrated about his role.

"Foursquare has a CEO; that job is taken" says a source briefed on Foursquare's founder divorce. "Might Naveen be happy about that or not, I don't know. I can see that as being hard."

Selvadurai worked hard to make Foursquare what it is today. But Foursquare has always been Crowley's dream. Crowley has been working on this company in some form for years, even before iPhones or his first startup Dodgeball existed.

Selvadurai, Foursquare PR, and Crowley declined to comment.

This is normal.

Cofounders and early employees at hot startups go through tough separations all the time.

Take Zynga, for example. When Mark Pincus realized employees with significant stock options were "resting and vesting," he gave them an ultimatum: get to work or get fired.

During Facebook's early days, Eduardo Saverin wasn't pulling his weight and Zuckerberg forced him out, diluting his stake in the company along the way.

The exception may be Google's Sergey Brin, who heads "special projects" while his cofounder, Larry Page, runs the company as CEO.

When cofounders do stick around, it's often in non-operating, lower-profile roles. Yahoo's David Filo still works in a cubicle, even though he's a billionaire. Apple's Steve Wozniak never wanted a say in running that company.

A source close to Selvadurai can't imagine him being happy with a role like Filo's.

"Naveen is a proud guy and a real entrepreneur -- someone who probably wouldn't want to remain in a situation that wasn't fulfilling for him," says a source.